MPLS Working Group Kamran Raza
Internet Draft Sami Boutros
Intended status: Standards Track
Expires: November 9, 2013 Cisco Systems
May 10, 2013
Disabling IPoMPLS and P2P PW LDP Application's State Advertisement
draft-ietf-mpls-ldp-ip-pw-capability-05.txt
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Abstract
Currently, no LDP capability is exchanged for LDP applications like
IP Label Switching and L2VPN P2P PW signaling. When an LDP session
comes up, an LDP speaker may unnecessarily advertise its local state
for such LDP applications even when the peer session is established
for some other applications like mLDP or ICCP. This document defines
a solution by which an LDP speaker announces to its peer its
disinterest in such non-negotiated applications. This, in turn,
disables the advertisement of corresponding application state, which
would have otherwise be advertised by default, over the established
LDP session.
Table of Contents
1. Introduction 3
2. Conventions used in this document 4
3. Non-negotiated LDP applications 4
3.1 Non-interesting State 5
4. Controlling State Advertisement for Non-negotiated LDP Apps 5
4.1 State Advertisement Control Capabilty 5
5.Capabilities Procedures 8
5.1 State Control Capability in an Initization message 8
5.2 State Control Capability in a Capabilty message 9
6. Operational Examples 9
6.1 Disabling IPoMPLS and P2P PW applications on an ICCP session 9
6.2 Disabling IPoMPLS application on a L2VPN/PW T-LDP session 10
6.3 Disabling IPoMPLS app. dynamically on an IP/PW LDP session 10
6.4 Disabling IPoMPLS application on an mLDP-only session 10
6.5 Disabling unwanted IP state advert. by an IP dual-stack LSR 11
7. Security Considerations 11
8. IANA Considerations 11
9. References 12
9.1 Normative References 12
9.2 Informative References 12
10. Acknowledgments 12
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1. Introduction
LDP Capabilities [RFC5561] introduced a mechanism to negotiate LDP
capabilities for a given feature between peer LSRs. The capability
mechanism insures that no unnecessary state is exchanged between peer
LSRs unless the corresponding feature capability is successfully
negotiated between the peers.
While new LDP features and applications, such as Typed Wildcard FEC
[RFC5918], Inter-Chassis Communication Protocol [ICCP], mLDP
[RFC6388], and L2VPN P2MP PW [P2MP-PW] make use of LDP capabilities
framework for their feature negotiation, the earlier LDP features and
applications like IP Label Switching and L2VPN P2P PW signaling
[RFC4447] [RFC4762] may cause LDP speakers to exchange application
state unnecessarily even when the given application is not enabled on
one of the LDP speakers participating in a given session. For
example, when bringing up and using an LDP peer session with a remote
PE LSR for purely ICCP signaling reasons, an LDP speaker may
unnecessarily advertise labels for IP (unicast) prefixes to this ICCP
related LDP peer.
Another example of unnecessary state advertisement can be cited when
LDP is to be deployed in an IP dual-stack environment. For instance,
an LSR that is locally enabled for both IPv4 and IPv6 label switching
may advertise label bindings for both IPv4 and IPv6 address families
towards an LDP peer that is interested in IPv4 prefix labels only. In
this case, the advertisement of IPv6 prefix labels to the peer is
unnecessary, as well as wasteful, from the point of view of LSR
memory/CPU and network resource consumption.
To avoid this unnecessary state advertisement and exchange, currently
an operator is typically required to configure and define some sort
of filtering policies on the LSR, which introduces operational
overhead and complexity for such deployments.
This document defines an LDP Capabilities [RFC5561] based solution by
which an LDP speaker may announce to its peer(s) its disinterest (or
non-support) for state related to IP Label Switching and/or L2VPN P2P
PW Signaling application at the time of session establishment. This
helps in avoiding unnecessary state advertisement for such feature
applications. The document also states the mechanics to dynamically
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disable or enable the state advertisement for such applications
during the session lifetime. The non-interesting state of an
application depends on the type of application and is described later
in section 3.1.
2. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
The term "IP" in this document refers to both IPv4 and IPv6 unicast
address families.
This document uses shorthand terms "IPoMPLS" to refer to IP Label
Switching application, and "P2P PW" to refer to L2VPN PW signaling
for FEC 128 and FEC 129 P2P Pseudowires.
3. Non-negotiated LDP applications
For the applications that existed prior to the definition of LDP
Capabilities framework [RFC5561], an LDP speaker typically
advertises, without waiting for any capabilities exchange and
negotiation, its corresponding application state to its peers right
after the session establishment. These early LDP applications
include:
o IPv4/IPv6 Label Switching ("IPoMPLS")
o L2VPN P2P PW signaling ("P2P PW")
To disable unnecessary state advertisement for such LDP applications
over an established LDP session, a new capability is introduced in
this document. This new capability controls the advertisement of
application state and enables an LDP speaker to notify its peer its
disinterest in the state of one or more of these "Non-negotiated"
LDP applications at the time of session establishment. Upon receipt
of such capability, the receiving LDP speaker, if supporting the
capability, disables the advertisement of the state related to the
application towards the sender. This new capability can also be sent
later in a Capability message to either disable a previously enabled
application's state advertisement or to enable a previously disabled
application's state advertisement.
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3.1. Non-interesting State
So far, this document has used the term application "state" to
generically refer to some non-interesting state. Now, let us further
specify and clarify this term:
. A non-interesting state of a non-negotiated application refers
to the application state which is of a no interest to an LSR
and need not be advertised to the LSR;
. This state MUST NOT be advertised in any of the LDP protocol
messages;
. This state is dependent on application type and specified
accordingly.
For IPoMPLS application type, the non-interesting state refers to
any state related to IP Prefix FEC (such as label bindings, LDP
Status). This document, however, does not classify IP address
bindings as a non-interesting state and allows the advertisement of
IP Address bindings to facilitate other LDP applications (such as
mLDP) that depend on learning of peer addresses over an LDP session
for their correct operation.
For P2P PW application type, the non-interesting state refers to any
state related to P2P PW FEC (such as label bindings, MAC [address]
withdrawal, and LDP PW Status).
From now onward in this document, the term "state" will mean to
refer to the "non-interesting state" for an application, as defined
in this section.
4. Controlling State Advertisement for Non-negotiated LDP Applications
To control advertisement of non-interesting state related to non-
negotiated LDP applications, namely IPoMPLS and P2P PW signaling, a
new capability TLV is defined as follows.
4.1. State Advertisement Control Capability
The "State Advertisement Control Capability" is a new Capability
Parameter TLV defined in accordance with section 3 of LDP
Capabilities specification [RFC5561]. The format of this new TLV is
as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| State Adv. Ctrl Cap.(IANA)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved | |
+-+-+-+-+-+-+-+-+
| |
~ State Advertisement Control Element(s) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format of an "State Advertisement Control Capability" TLV
The value of the U-bit for the TLV MUST be set to 1 so that a
receiver MUST silently ignore this TLV if unknown to it, and
continue processing the rest of the message. Whereas, The value of
F-bit MUST be set to 0. Once advertised, this capability cannot be
withdrawn; thus S-bit MUST be set to 1 both in an Initialization and
Capability message.
The capability data associated with this State Advertisement Control
(SAC) Capability TLV is one or more State Advertisement Control
(SAC) Elements, where each element indicates enabling/disabling of
advertisement of non-interesting state for a given application. The
format of a SAC Element is defined as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| State |D|Rsvd1| Rsvd2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Format of an "State Advertisement Control Element"
Where:
State: Defines the type of application state (to be controlled).
The value of this field is defined as follows:
1: IPv4 Label switching
2: IPv6 Label switching
3: P2P PW FEC128 signaling
4: P2P PW FEC129 signaling
0, 5-15: Reserved.
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D bit: Controls the advertisement of the state:
1: Disable state advertisement
0: Enable state advertisement
When sent in an Initialization message, D bit MUST be set to 1.
Rsvd1, Rsvd2: Reserved for future use. MBZ on transmit and ignored
on receipt.
The "Length" field of SAC Capability TLV depends on the number of
SAC Elements present in the TLV. For example, if there are two
elements present, then the Length field is set to 5 octets. A
receiver of this capability TLV can deduce number of elements
present in the TLV by using the Length field.
From now onward, this document uses the term "element" to refer to a
SAC Element.
As described earlier, SAC Capability TLV MAY be included by an LDP
speaker in an Initialization message to signal to its peer LSR that
state advertisement for one or more application(s) need to be
disabled on the given peer session. This TLV can also be sent later
in a Capability message to selectively enable or disable these
applications. A SAC Capability TLV MUST contain elements with
distinct state types and the TLV MUST NOT contain the same state
type more than once. If a receiver receives such a malformed TLV, it
SHOULD discard this TLV and continue processing rest of the message.
To control more than one application state, a sender LSR can either
send a single capability TLV in a message with multiple elements
present, or can send separate messages with capability TLV
specifying one or more elements. A receiving LSR, however, MUST
treat each incoming capability TLV for a given application state
type as an update to its existing policy for the given type.
To understand capability updates from an example, let us consider 2
LSRs, S (LDP speaker) and P (LDP peer), both of which support all
the non-negotiated applications listed earlier. By default, these
LSR will advertise state for these applications, as configured, to
their peer as soon as an LDP session is established. Now assume that
P receives from S a SAC capability in the Initialization message
with "IPv6 Label switching" and "P2P PW FEC129" states disabled.
This updates P's outbound policy towards S to advertise state
related to only "IPv4 Label switching" and "P2P PW FEC 128"
applications. Later, P receives another capability update from S
via a Capability message with "IPv6 Label switching" enabled and
"P2P PW FEC128" disabled. This results in P's outbound policy
towards S to advertise both IPv4 and IPv6 Label switching state, and
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disable both P2P PW FEC128 and FEC 129 signaling. Finally, P
receives another update from S via a Capability message that
specifies to disable all four non-negotiated applications state,
resulting in P outbound policy towards S to block/disable state for
all these applications, and only advertise state for any other
application, as applicable.
5. Capabilities Procedures
The SAC capability conveys the desire of an LSR to disable the
receipt of unwanted/unnecessary state from its LDP peer. This
capability is uni-lateral and uni-directional in nature, and a
receiving LSR is not required to send a similar capability TLV in an
Initialization or Capability message towards the sender of this
capability. This unilateral behavior conforms to the procedures
defined in the Section 6 of LDP Capabilities [RFC5561].
After this capability is successfully negotiated (i.e. sent by an
LSR and received/understood by its peer), then the receiving LSR
MUST NOT advertise any state related to the disabled applications
towards the capability sending LSR until and unless these
application states are explicitly enabled again via a capability
update. Upon receipt of a capability update to disable an enabled
application [state] during the lifetime of a session, the receiving
LSR MUST also withdraw from the peer any previously advertised state
(corresponding to the disabled application).
If a receiving LDP speaker does not understand the SAC capability
TLV, then it MUST respond to the sender with "Unsupported TLV"
notification as described in LDP Capabilities [RFC5561]. If a
receiving LDP speaker does not understand or does not support an
application specified in an application control element, it SHOULD
silently ignore/skip such an element and continue processing rest of
the TLV.
5.1. State Control Capability in an Initialization message
LDP Capabilities [RFC5561] framework dictates that the S-bit of
capability parameter in an Initialization message MUST be set to 1
and SHOULD be ignored on receipt.
An LDP speaker determines (e.g. via some local configuration or
default policy) if it needs to disable IPoMPLS and/or P2P PW
applications with a peer LSR. If there is a need to disable, then
the SAC TLV needs to be included in the Initialization message with
respective SAC elements included with their D bit set to 1.
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An LDP speaker that supports the SAC capability MUST interpret the
capability TLV in a received Initialization message such that it
disables the advertisement of the application state towards the
capability sending LSR for IPoMPLS and/or P2P PW applications if
their SAC element's D bit is set to 1.
5.2. State Control capability in a Capability message
If the LDP peer supports "Dynamic Announcement Capability"
[RFC5561], then an LDP speaker may send SAC capability in a
Capability message towards the peer. Once advertised, these
capabilities cannot be withdrawn and hence the S-bit of the TLV MUST
be set to 1 when sent in a Capability message.
An LDP speaker may decide to send this TLV towards an LDP peer if
one or more of its IPoMPLS and/or P2P PW signaling applications get
disabled, or if previously disabled application gets enabled again.
In this case, the LDP speaker constructs the TLV with appropriate
SAC element(s) and sends the corresponding capability TLV in a
Capability message.
Upon receipt of this TLV in a Capability message, the receiving LDP
speaker reacts in the same manner as it reacts upon the receipt of
this TLV in an Initialization message. Additionally, the peer
withdraws/advertises the application state from/to the capability
sending LDP speaker according to the capability update.
6. Operational Examples
6.1. Disabling IPoMPLS and P2P PW applications on an ICCP session
Consider two PE routers, LSR1 and LSR2, which understand/support SAC
capability TLV, and have an established LDP session to exchange ICCP
state related to dual-homed devices connected to these LSRs. Let us
assume that both LSRs are provisioned not to exchange any state for
IPoMPLS (IPv4/IPv6) and P2P PW (FEC128/129) application.
To indicate their disinterest in these applications, the LSRs will
include a SAC capability TLV (with 4 SAC elements corresponding to
these 4 applications with D bit set to 1 for each one) in the
Initialization message. Upon receipt of this TLV in Initialization
message, the receiving LSR will disable the advertisement of
IPv4/IPv6 label bindings, as well as P2P PW FEC128/129 signaling,
towards its peer after session establishment.
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6.2. Disabling IPoMPLS application on a L2VPN/PW T-LDP session
Now, consider LSR1 and LSR2 have an established T-LDP session for
P2P PW application to exchange label bindings for FEC 128/129. Given
that there is no need to exchange IP (v4/v6) label bindings between
the PE LSRs over a PW T-LDP session in most typical deployments, let
us assume that LSRs are provisioned to disable IPoMPLS (IPv4/IPv6)
application state on given PW session.
To indicate their disinterest in IPoMPLS application over a PW T-LDP
session, the LSRs will follow/apply the same procedures to disable
IPv4 and IPv6 label switching as described in previous section. As a
result, only P2P PW related state will be exchanged between these
LSRs over this T-LDP session.
6.3. Disabling IPoMPLS application dynamically on an established IP/PW
LDP session
Assume that LSRs from previous sections were initially provisioned
to exchange both IPoMPLS and P2P PW state over the session between
them, and also support "Dynamic Announcement" Capability [RFC5561].
Now, assume that LSR1 is dynamically provisioned to disable IPoMPLS
(IPv4/IPv6) over T-LDP session with LSR2. In this case, LSR1 will
send SAC capability TLV in a Capability message towards LSR2 with
application control elements defined for IPv4 and IPv6 label
switching with D bit set to 1. Upon receipt of this TLV, LSR2 will
disable IPoMPLS application state(s) towards LSR1 and withdraw all
previous application state from LSR1. To withdraw label bindings
from its peer, LSR2 MAY use a single Prefix FEC Typed Wildcard Label
Withdraw message [RFC5918].
This dynamic disability of IPoMPLS application does not impact L2VPN
P2P PW application on the given session, and both LSRs should
continue to exchange PW Signaling application related state.
6.4. Disabling IPoMPLS application on an mLDP-only session
Now assume that LSR1 and LSR2 have formed an LDP session to exchange
mLDP application state only. In typical deployments, LSR1 and LSR2
also exchange label bindings for IP prefixes over an mLDP session,
which is unnecessary and wasteful for an mLDP-only LSR.
Using the procedures defined earlier, an LSR can indicate its
disinterest in IPoMPLS application state to its peer upon session
establishment time or dynamically later via LDP capabilities update.
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Reference to section 3.1, the peer disables the advertisement of any
state related to IP Prefix FECs, but still advertises IP address
bindings that are required for the correct operation of mLDP.
6.5. Disabling unwanted IP state advertisement by an IP dual-stack LSR
In IP dual-stack scenarios, an LSR2 may advertise unnecessary state
(e.g. IPv6 prefix label bindings) towards peer LSR1 corresponding to
IPv6 label switching application once a session is established
mainly for exchanging state for IPv4. The similar scenario also
applies when advertising IPv4 label switching state on a session
meant for IPv6. The SAC capability and its procedures defined in
this document can help to avoid such unnecessary state
advertisement.
Consider IP dual-stack environment where LSR2 is enabled for IPoMPLS
application for both IPv4 and IPv6, but LSR1 is enabled for (or
interested in) only IPv4oMPLS Label switching. To avoid receiving
unwanted state advertisement for IPv6oMPLS Label switching
application from LSR2, LSR1 can send SAC capability with element for
IPv6 label switching with D bit set to 1 in the Initialization
message towards LSR2 at the time of session establishment. Upon
receipt of this capability, LSR2 will disable all IPv6 label binding
advertisement towards LSR1. If IPv6oMPLS Label switching application
is later enabled on LSR1, LSR1 can update the capability by sending
SAC capability in a Capability message towards LSR2 to enable
IPv6oMPLS Label switching application dynamically.
7. Security Considerations
The proposal introduced in this document does not introduce any new
security considerations beyond that already apply to the base LDP
specification [RFC5036] and [RFC5920].
8. IANA Considerations
This document defines a new LDP cpability parameter TLV. IANA is
requested to assign the lowest available value after 0x0500 from
"TLV Type Name Space" in the "Label Distribution Protocol (LDP)
Parameters" registry within "Label Distribution Protocol (LDP)
Name Spaces" as the new code point for the LDP TLV code point.
+-----+---------------------+---------------+-----------------------+
|Range| Description | Reference |Notes/Registration Date|
+-----+---------------------+---------------+-----------------------+
| TBA | State Advertisement | This document | |
| | Control Capability | | |
+-----+---------------------+---------------+-----------------------+
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9. References
9.1. Normative References
[RFC5036] L. Andersson, I. Minei, and B. Thomas, "LDP
Specification", RFC 5036, September 2007.
[RFC5561] B. Thomas, K. Raza, S. Aggarwal, R. Aggarwal, and JL. Le
Roux, "LDP Capabilities", RFC 5561, July 2009.
[RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
9.2. Informative References
[RFC5918] R. Asati, I. Minei, and B. Thomas, "Label Distribution
Protocol Typed Wildcard FEC", RFC 5918, August 2010.
[RFC4447] L. Martini, E. Rosen, El-Aawar, T. Smith, and G. Heron,
"Pseudowire Setup and Maintenance using the Label
Distribution Protocol", RFC 4447, April 2006.
[RFC4762] M. Lasserre, and V. Kompella, "Virtual Private LAN Service
(VPLS) Using Label Distribution Protocol (LDP) Signaling",
RFC 4762, January 2007.
[P2MP-PW] Martini, L. et. al, "Signaling Root-Initiated Point-to-
Multipoint Pseudowires using LDP", draft-ietf-pwe3-p2mp-pw-
04.txt, Work in Progress, March 2012.
[ICCP] L. Martini, S. Salam, A. Sajassi, and S. Matsushima,
"Inter-Chassis Communication Protocol for L2VPN PE
Redundancy", draft-ietf-pwe3-iccp-11.txt, Work in
Progress, February 2013.
[RFC6388] I. Minei, I. Wijnands, K. Kompella, and B. Thomas, "LDP
Extensions for P2MP and MP2MP LSPs", RFC 6388, November
2011.
[RFC5920] L. Fang, et al., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
10. Acknowledgments
The authors would like to thank Eric Rosen for his valuable input
and comments. We also acknowledge Karthik Subramanian and IJsbrand
Wijnands for bringing up mLDP use case.
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This document was prepared using 2-Word-v2.0.template.dot.
Authors' Addresses
Kamran Raza
Cisco Systems, Inc.,
2000 Innovation Drive,
Ottawa, ON K2K-3E8, Canada.
E-mail: skraza@cisco.com
Sami Boutros
Cisco Systems, Inc.
3750 Cisco Way,
San Jose, CA 95134, USA.
E-mail: sboutros@cisco.com
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